VLSM Fundamentals

VLSM Fundamentals

Introduction

Objective

The primary goal of this page is to provide a simple explanation to the techniques for designing and planning LANs using VLSM. The materials presented here are designed to help a person who may be at the first course level in the Networking Academy CCNA Exploration curriculum. Knowing the methods explained here will help establish a firm understanding needed as you prepare for your CCNA certification.

Definitions

VLSM - Variable-length subnet masking. This is a more practical way of subnetting a network by efficiently using the Host bits in the subnet.

CIDR -

Typical Classful Networks

An 'older' method, wasteful use of IP addresses, not efficient use of IP address assignments within networks.

Remember that when you subnet a classful network all of the networks will have the same number of hosts because they have the same subnet mask.

Basic CIDR understanding

There are also a range of IP addresses you can only used privately, not on the internet.

These are the RFC 1918 private IP's:

Class A 10.0.0.0 to 10.255.255.255

Class B 172.16.0.0 to 172.32.255.255

Class C 192.168.0.0 to 192.168.255.255

1 examine the number of bits borrowed for Host addresses.

2 observe the most interesting octet. This will allow you to determine the subnet mask.

3 determine the network increment (size of the network blocks) 256 - subnet mask = block size.

the number of Networks is figured out by 2 to the Nth power - 2 ( N standing for the number of network bits borrowed).

Remember that the number of networks cannot include all 0's or 1's. All 0's is the network address and the broadcast address is all 1's.

Steps to subnet in your head

1

2...

The following

• Postfix
• Send mail

VLSM Design

VLSM is the process where you are subnetting a subnet using varying subnet masks.

Your IP plan will determine the basic layout of networks. Multiple solutions are possible.

Requires forethought and planning. Current and future networks.

IP Subnet Zero.

Implementing VLSM

Make sure that your network assignments and subnet masks are accurate and that they fit your topology.

Verify that your IP addresses and subnet masks to not crossover any binary and octal bit boundaries. If they are incorrect, you will receive an error when configuring router interfaces.